transcendental-lisp/src/function/builtin/COND.java

package function.builtin;

import function.*;
import sexpression.*;

public class COND extends LispFunction {

    private ArgumentValidator argumentValidator;

    public COND() {
        this.argumentValidator = new ArgumentValidator("COND");
        this.argumentValidator.setEveryArgumentExpectedType(Cons.class);
        this.argumentValidator.doNotAcceptNil();
    }

    public SExpression call(Cons argumentList) {
        argumentValidator.validate(argumentList);

        return callTailRecursive(argumentList);
    }

    private SExpression callTailRecursive(Cons argumentList) {
        if (argumentList.nullp())
            return Nil.getInstance();

        Cons clause = (Cons) argumentList.getCar();
        Cons remainingClauses = (Cons) argumentList.getCdr();
        SExpression test = EVAL.eval(clause.getCar());

        if (isTestSuccessful(test))
            return evaluateResult(clause, test);

        return callTailRecursive(remainingClauses);
    }

    private boolean isTestSuccessful(SExpression test) {
        return test != Nil.getInstance();
    }

    private SExpression evaluateResult(Cons clause, SExpression test) {
        SExpression lastResultValue = test;

        for (SExpression result = clause.getCdr(); result.consp(); result = advanceCons(result))
            lastResultValue = EVAL.eval(getCar(result));

        return lastResultValue;
    }

    private SExpression advanceCons(SExpression knownCons) {
        return ((Cons) knownCons).getCdr();
    }

    private SExpression getCar(SExpression knownCons) {
        return ((Cons) knownCons).getCar();
    }

    public boolean evaluateArguments() {
        return false;
    }

}